Virtualization - Jul 2023 - [PATCH net-next v4 2/2] virtio-net: add cond_resched() to the command waiting loop

On 7/20/23 1:38 AM, Jason Wang wrote:
> 
> Adding cond_resched() to the command waiting loop for a better
> co-operation with the scheduler. This allows to give CPU a breath to
> run other task(workqueue) instead of busy looping when preemption is
> not allowed on a device whose CVQ might be slow.
> 
> Signed-off-by: Jason Wang <jasowang at redhat.com>
This still leaves hung processes, but at least it doesn't pin the CPU 
any more.  Thanks.

Reviewed-by: Shannon Nelson <shannon.nelson at amd.com>

> ---
>   drivers/net/virtio_net.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 9f3b1d6ac33d..e7533f29b219 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2314,8 +2314,10 @@ static bool virtnet_send_command(struct virtnet_info
*vi, u8 class, u8 cmd,
>           * into the hypervisor, so the request should be handled
immediately.
>           */
>          while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> -              !virtqueue_is_broken(vi->cvq))
> +              !virtqueue_is_broken(vi->cvq)) {
> +               cond_resched();
>                  cpu_relax();
> +       }
> 
>          return vi->ctrl->status == VIRTIO_NET_OK;
>   }
> --
> 2.39.3
> 
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> Virtualization mailing list
> Virtualization at lists.linux-foundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/virtualization

On Thu, Jul 20, 2023 at 01:26:20PM -0700, Shannon Nelson
wrote:
> On 7/20/23 1:38 AM, Jason Wang wrote:
> > 
> > Adding cond_resched() to the command waiting loop for a better
> > co-operation with the scheduler. This allows to give CPU a breath to
> > run other task(workqueue) instead of busy looping when preemption is
> > not allowed on a device whose CVQ might be slow.
> > 
> > Signed-off-by: Jason Wang <jasowang at redhat.com>
> 
> This still leaves hung processes, but at least it doesn't pin the CPU
any
> more.  Thanks.
> Reviewed-by: Shannon Nelson <shannon.nelson at amd.com>
> 
I'd like to see a full solution
1- block until interrupt
2- still handle surprise removal correctly by waking in that case


> > ---
> >   drivers/net/virtio_net.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 9f3b1d6ac33d..e7533f29b219 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -2314,8 +2314,10 @@ static bool virtnet_send_command(struct
virtnet_info *vi, u8 class, u8 cmd,
> >           * into the hypervisor, so the request should be handled
immediately.
> >           */
> >          while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> > -              !virtqueue_is_broken(vi->cvq))
> > +              !virtqueue_is_broken(vi->cvq)) {
> > +               cond_resched();
> >                  cpu_relax();
> > +       }
> > 
> >          return vi->ctrl->status == VIRTIO_NET_OK;
> >   }
> > --
> > 2.39.3
> > 
> > _______________________________________________
> > Virtualization mailing list
> > Virtualization at lists.linux-foundation.org
> > https://lists.linuxfoundation.org/mailman/listinfo/virtualization